Average Charge State Research Articles

Intensity-dependent characteristics of a picosecondlaser-produced Cu plasma

The interaction of a 1 ps laser pulse of moderate contrast ratio(~104) with a Cu target is investigated at laser intensities I of upto 3×1016 W cm-2 with the use of corpuscular, x-ray and opticaldiagnostics. Intensity-dependent characteristics of ion and x-ray emission aswell as reflectivity, electron temperature and average charge state of plasmaproduced by the laser pulse are presented and discussed. It is found that(a) the specular reflectivity dependence on intensity for a moderate-contrastultrashort (⩽1 ps) pulse is qualitatively and quantitatively differentfrom that for a high-contrast ultrashort pulse; (b) the scaling laws forparameters of thermal ions are essentially different from those for fast ions;(c) the soft x-ray yield scales as Iα with α≈2-2.5 inthe intensity range 1014-3×1016 W cm-2; and (d) electrontemperature scales as I0.44 at 1015<I<3×1016 W cm-2.

Energetics of vacuum arc cathode spots

Vacuum arcs need to generate the current-carrying plasma at cathode spots. The cohesive energy of the cathode material determines the energy the arc needs to provide for the phase transitions of the cathode material. As recent experiments confirm, the greater the cohesive energy the higher the burning voltage (“cohesive energy rule”). This relation is founded in the energy balance of cathode spot operation, regardless of the specific cathode mechanisms involved. A greater power input, as determined by the cohesive energy via the cohesive energy rule, leads to enhanced power output in various forms such as enhanced electron temperature, ion charge states, and ion velocity. Using the Cohesive Energy Rule, secondary relations such as the well known relation between boiling point of the cathode and average ion charge state can be explained.

Possible impact of multi-electron loss events on the average beam charge state in an HIF target chamber and a neutral beam approach

Experiments were carried out during the early 1980s to assess the obtainable atomic neutralization of energetic beams of negative ions ranging from lithium to silicon. The experiments found (Grisham et al. Rev. Sci. Instrum. 53 (1982) 281; Princeton Plasma Physics Laboratory Report PPPL-1857, 1981) that, for higher atomic number elements than lithium, it appeared that a substantial fraction of the time more than one electron was being lost in a single collision. This result was inferred from the existence of more than one ionization state in the product beam for even the thinnest line densities at which any electron removal took place. Because of accelerator limitations, these experiments were limited to maximum energies of 7 MeV. However, based upon these results, it is possible that multi-electron loss events may also play a significant role in determining the average ion charge state of the much higher Z and more energetic beams traversing the medium in an heavy ion fusion chamber. This could result in the beam charge state being considerably higher than previously anticipated, and might require designers to consider harder vacuum ballistic focusing approaches, or the development of additional space charge neutralization schemes. This paper discusses the measurements that gave rise for these concerns, as well as a description of further measurements that are proposed to be carried out for atomic numbers and energies per amu which would be closer to those required for heavy ion fusion drivers. With a very low current beam of a massive, but low charge state energetic ion, the charge state distribution emerging from a target gas cell could be measured as a function of line density and medium composition. Varying the line density would allow one to simulate the charge state evolution of the beam as a function of distance into the target chamber. This paper also briefly discusses a possible alternative driver approach using photodetachment-neutralized atomic beams, which could reduce plasma complications far from the target, but which would impose more stringent limitations upon chamber pressure and repetition rate.

Multiple electron stripping of 3.4 MeV/amu Kr7+ and Xe11+ in nitrogen

Use of heavy ions beams with ∼10 MeV/amu mass ∼200, and average charge state of 1+ has been proposed as a driver for heavy ion fusion. Stripping of the ion beam by background gas can lead to an increase in the space charge density of the beam, which may make focusing the intense ion beam onto small targets more complex. Knowledge of the electron loss cross sections is essential to understand and address the problem. Currently, there are no 10 MeV/amu mass=200, charge state=1 beams available, and the theories that calculate electron loss cross sections can be experimentally tested only by using available beams of somewhat lower energy and higher initial charge state. The charge state distribution of ions produced in single collisions of 3.4 MeV/amu Kr7+ and 3.4 MeV/amu Xe11+ in N2 have been measured at the Texas A&amp;M Cyclotron Institute using a windowless gas cell. The charge states of the outgoing ions are determined by magnetic analysis using a position-sensitive microchannel-plate detector. The cross sections for single and multiple electron loss are determined, and the results indicate that substantial multiple-electron loss occurs. The relative cross section for loss of i+1 electrons is 0.3–0.7 times that for i electron loss. The average number of electrons removed per one collision (sum of the electron-weighted cross sections normalized to the total cross section) is 1.86 for Kr and 1.97 for Xe.

Supercharged protein and peptide ions formed by electrospray ionization.

The multiple charging of large molecules in electrospray ionization provides key advantages for obtaining accurate molecular weights by mass spectrometry and for obtaining structural information by tandem mass spectrometry and MS(n) experiments. Addition of glycerol or m-nitrobenzyl alcohol into the electrospray solutions dramatically increases both the maximum observed charge state and the abundances of the high charge states of protein and peptide ions. Adding glycerol to acidified aqueous solutions of cytochrome c shifts the most abundant charge state from 17+ to 21+, shifts the maximum charge state from 20+ to 23+, and shifts the average charge state from 16.6+ to 20.9+. Much less m-nitrobenzyl alcohol (<1%) is required to produce similar results. With just 0.7% m-nitrobenzyl alcohol, even the 24+ charge state of cytochrome c is readily observed. Similar results are obtained with myoglobin and (Lys)4. For the latter molecule, the 5+ charge state is observed in the electrospray mass spectrum obtained from solutions containing 6.7% m-nitrobenzyl alcohol. This charge state corresponds to protonation of all basic sites in this peptide. Although the mechanism for enhanced charging is unclear, it does not appear to be a consequence of conformational changes of the analyte molecules. This method of producing highly charged protein ions should be useful for improving the performance of mass measurements on mass spectrometers with performances that decrease with increasing m/z. This should also be particularly useful for tandem mass spectrometry experiments, such as electron capture dissociation, for which highly charged ions are desired.

Correlation Between the Impurity Content, the Average Charge State of Chromium Cations and Optical Absorption in Sillenite Crystals Doped by Chromium in Wide Range of Concentrations

Single crystals of sillenites (Bi 12 SiO 20 , Bi 12 TiO 20 ) doped with chromium in a wide range of concentrations from 1 x 10 -3 up to 1.8 x 10 -2 wt. % were grown by the Czochralski and top-seed solution growth (TSSG) techniques. To estimate the content and the average charge state of chromium in the grown crystals the chemical analysis by the modified diphenylcarbazide method was applied. The dependencies of the chromium distribution coefficient, the average charge state of Cr cations, and optical absorption on the Cr concentration were found. Both the shift of the absorption edge toward lower frequencies and the appearance of an additional absorption band in the near IR were observed when the chromium concentration in the crystals was increased. The experimental data suggest that chromium has at least two charge states and occupies probably different positions in the sillenite unit cell.

Transport of Hollow Atoms Through Thin Dielectric Films

We have determined exit charge states and energy losses of slow, highly charged gold ions after transmission of thin (7 ± 3 nm) diamond like carbon foils. Average charge states of up to 37+ were observed for Th80+ at 8.2 keV/u (0.57 vBohr). The final charge state is populated through re-arrangement processes after the projectile has left the foil. High final charge states indicate that none of the M-shell vacancies in Th80+ could be filled inside the foil. The increase of the average exit charge state as a function of M-shell vacancies is a sign of strongly enhanced X-ray fluorescent yields for hollow atom decay in vacuum.

Conductivities in Hot Aluminium Plasma

We perform model calculations for the electrical and thermal conductivity of aluminium plasma within the generalized linear response method of Zubarev for temperatures of (5–25) eV and densities of (0.01–10) g/cm3. The composition in the expanded plasma region is determined by considering higher ionization states up to 5+ and solving the respective system of coupled mass action laws. Besides this chemical picture, a generalized Thomas-Fermi model is applied to calculate the equation of state and the average charge state of the ions for densities near and above solid state density. Interactions between the various species are treated on T matrix level. Numerical results for the electrical and thermal conductivity of aluminium plasma are compared with experimental data and, for high densities, also with results of a Born approximation with respect to a weak electron-ion pseudopotential.

Composition of quasi‐stationary solar wind flows from Ulysses/Solar Wind Ion Composition Spectrometer

Using improved, self‐consistent analysis techniques, we determine the average solar wind charge state and elemental composition of nearly 40 ion species of He, C, N, O, Ne, Mg, Si, S, and Fe observed with the Solar Wind Ion Composition Spectrometer on Ulysses. We compare results obtained during selected time periods, including both slow solar wind and fast streams, concentrating on the quasi‐stationary flows away from recurrent or intermittent disturbances such as corotating interaction regions or coronal mass ejections. In the fast streams the charge state distributions are consistent with a single freezing‐in temperature for each element, whereas in the slow wind these distributions appear to be composed of contributions from a range of temperatures. The elemental composition shows the well‐known first ionization potential (FIP) bias of the solar wind composition with respect to the photosphere. However, it appears that our average enrichment factor of low‐FIP elements in the slow wind, not quite a factor of 3, is smaller than that in previous compilations. In fast streams the FIP bias is found to be yet smaller but still significantly above 1, clearly indicating that the FIP fractionation effect is also active beneath coronal holes from where the fast wind originates. This imposes basic requirements upon FIP fractionation models, which should reproduce the stronger and more variable low‐FIP bias in the slow wind and a weaker (and perhaps conceptually different) low‐FIP bias in fast streams. Taken together, these results firmly establish the fundamental difference between the two quasi‐stationary solar wind types.

Ion velocities in vacuum arc plasmas

Ion velocities in vacuum arc plasmas have been measured for most conducting elements of the Periodic Table. The method is based on drift time measurements via the delay time between arc current modulation and ion flux modulation. A correlation has been found between the element-specific ion velocity and average ion charge state; however, differently charged ions of the same element have approximately the same velocity. These findings contradict the potential hump model but are in agreement with a gasdynamic model that describes ion acceleration as driven by pressure gradients and electron-ion friction. The differences between elements can be explained by the element-specific power density of the cathode spot plasma which in turn determines the temperature, average charge state, and ion velocity of the expanding vacuum arc plasma.

Use of Fe charge state changes as a tracer for solar wind entry to the magnetosphere

We present a comparison of high charge state ion composition measurements from ACE located near the L1 point and POLAR in the mid‐altitude cusp/cleft, during the 13 May 1998 CME event. Using the frozen in charge state of the solar wind origin Fe ions as a tracer, we demonstrate the connectivity between the ACE observations and the high latitude measurements from POLAR. The ion charge state distributions were observed to show abrupt changes with variations in the average charge state of Fe between +6 and +15 corresponding to changes in the coronal source temperature of 4×105 to 2×106 K. A good correlation between the measurements at L1 and in the cusp/cleft was found, indicating direct entry of solar wind material during periods of southward IMF. In addition there is possible evidence for delayed entry during a period of predominantly northward IMF that may indicate a less direct entry mechanism for this configuration.

Multiple ionization of argon in coincidence with projectile ions in 60–120 MeV Si q+-Ar collisions

A projectile ion-recoil ion coincidence technique has been employed to study the multiple ionization and the charge transfer processes in collisions of 60–120 MeV Si q+ (q = 4−14) ions with neutral argon atoms. The relative contribution of different ionization channels, namely; direct ionization, electron capture and electron loss leading to the production of slow moving multiply charged argon recoil ions have been investigated. The data reported on the present collision system result from a direct measurement in the considered impact energy for the first time. The total ionization cross-sections for the recoil ions are shown to scale as q 1.7/E 0.5 , where E p is the energy in MeV of the projectile and q its charge state. The recoil fractions for the cases of total- and direct ionizations are found to decrease with increasing recoil charge state j. The total ionization fractions of the recoils are seen to depend on q and to show the presence of a ‘shell-effect’ of the target. Further, the fractions are found to vary as 1/j 2 upto j = 8+. The average recoil charge state 〈j〉 increases slowly with q and with the number of lost or captured electrons from or into the projectile respectively. The projectile charge changing cross-sections σ qq′ are found to decrease with increasing q for loss ionization and to increase with q for direct-and capture ionization processes respectively. The physics behind various scaling rules that are found to follow our data for different ionization processes is reviewed and discussed.

Temporal development of the plasma composition of a pulsed aluminum plasma stream in the presence of oxygen

We describe the temporal development of the plasma composition of pulsed aluminum plasma streams at various oxygen pressures. The plasma was formed with a vacuum arc plasma source and the time resolved plasma composition was measured with time-of-flight charge-to-mass spectrometry. The temporal development of the plasma composition as well as the Al average ion charge state was found to be a strong function of the oxygen pressure. Oxygen and hydrogen concentrations of up to 0.36 and 0.32, respectively, were found in the first 50 μs of the pulse at oxygen pressures of ⩾5×10−5 Torr. The average charge state of aluminum ions was found to vary from +1.2 to +2.5 depending on the oxygen pressure and the time elapsed after ignition of the arc. These results are of fundamental importance for the understanding of the evolution of the composition (through the plasma composition) and microstructure (through the Al ion flux energy) of alumina thin films produced by pulsed, reactive aluminum plasmas.

Interaction between explosive and analyte layers in explosive matrix-assisted plasma desorption mass spectrometry.

An HMX/insulin two-layer system was chosen as a model for further investigation of the matrix properties of explosive materials for protein analytes in plasma desorption mass spectrometry. The dependencies of the molecular ion yield and average charge state as a function of the analyte thickness were studied. An increase in the charge state of multiply protonated molecular species was confirmed as the major matrix effect, with the average charge state z at the smallest thickness studied being higher than in matrix-assisted laser desorption/ionization and closer to the value obtained in electrospray ionization under standard acidic conditions. Observed charge state distributions are significantly narrower than the corresponding Poisson distributions, which suggests that the protonation of insulin is limited in plasma desorption by the number of basic sites in the molecule, similar to electrospray ionization. Both the curve displaying total molecular ion yield and the one showing the total charge (proton) yield as a function of the insulin thickness have maxima at a thickness different from an insulin monolayer. These observations diminish the significance of a matrix/analyte interface mechanism for the explosive matrix assistance. Instead, a mechanism related to the chemical energy release during conversion of the explosive after the ion impact is proposed. As additional mechanisms, enhanced protonation of the analyte through collisions with products of the explosive decay is considered, as well as electron scavenging by other products, which leads to a higher survival probability of positively charged protein molecular ions. Copyright 1999 John Wiley & Sons, Ltd.

Interaction between explosive and analyte layers in explosive matrix‐assisted plasma desorption mass spectrometry

An HMX/insulin two-layer system was chosen as a model for further investigation of the matrix properties of explosive materials for protein analytes in plasma desorption mass spectrometry. The dependencies of the molecular ion yield and average charge state as a function of the analyte thickness were studied. An increase in the charge state of multiply protonated molecular species was confirmed as the major matrix effect, with the average charge state z at the smallest thickness studied being higher than in matrix-assisted laser desorption/ionization and closer to the value obtained in electrospray ionization under standard acidic conditions. Observed charge state distributions are significantly narrower than the corresponding Poisson distributions, which suggests that the protonation of insulin is limited in plasma desorption by the number of basic sites in the molecule, similar to electrospray ionization. Both the curve displaying total molecular ion yield and the one showing the total charge (proton) yield as a function of the insulin thickness have maxima at a thickness different from an insulin monolayer. These observations diminish the significance of a matrix/analyte interface mechanism for the explosive matrix assistance. Instead, a mechanism related to the chemical energy release during conversion of the explosive after the ion impact is proposed. As additional mechanisms, enhanced protonation of the analyte through collisions with products of the explosive decay is considered, as well as electron scavenging by other products, which leads to a higher survival probability of positively charged protein molecular ions. Copyright © 1999 John Wiley & Sons, Ltd.

Reduced charge state of MeV carbon cluster constituents exiting thin carbon foils

Direct determination of average charge states have been performed with an electrostatic analysis method for carbon ions and carbon cluster $({\mathrm{C}}_{n},$ $n=3$--10) constituents exiting thin carbon foils. The velocity of the mono and polyatomic projectiles was identical, ranging from 1--4 MeV per atom. The average charge state of carbon cluster constituents is significantly reduced as compared to that of a single carbon projectile at the same velocity, the larger the size of the cluster, the smaller the charge state per cluster atom. This lowering effect decreases when the foil thickness increases.

Average and maximum charge states of arginine-containing dendrimer-like peptide ions formed by electrospray ionization

The maximum and average charge states formed by electrospray ionization of dendrimer-like multiple antigenic peptides (MAPs) which differ in structure only in the presence of an arginine residue at the N-termini of their four peptide chains have been investigated. Stepwise addition of arginine residues leads to increased charging. It has been found that the average charge state is linearly correlated to the number of arginine residues which allows the conclusion that the four peptide chains are effectively independent. The average charge state zav is shifted with each added arginine residue by roughly 0.3 units towards lower m/z ratios. Modification of the alpha-amino groups by acetylation reduces zav as compared with the corresponding non-modified model peptides. This suggests that the N-terminal arginineis to some extent protonatedon both its alpha-amino group and its side-chain guanidino group. The Coulomb repulsion is presumably reduced through intramolecular charge solvation in the N-terminal part of the peptide chains.

Evaluation of heat-induced conformational changes in proteins by nanoelectrospray Fourier transform ion cyclotron resonance mass spectrometry

The characterization of heat-induced conformational changes of proteins by nanoelectrospray mass spectrometry is shown. The used device is the first to make a simple and effective investigation of the denaturation of proteins possible using minimum sample consumption, as enabled by nanoelectrospray mass spectrometry. As indicated by average charge states at several solution temperatures, a thermal induced denaturation of ubiquitin, in the temperature range between 25 and 90°C, was investigated. A single structure transition of ubiquitin was observed. Heating and unassisted cooling by air convection of the sample solution occurs directly inside the nanospray capillary during the electrospray measurement. Therefore, this new device is also suitable for probing for the reversibility of conformational changes in proteins.

Effect of ionization and recombination coefficients on the charge-state distribution of ions in laser-produced aluminum plasmas

Various formulations for the ionization and recombination coefficients reported and used in the literature have been compared for laser-produced Al plasmas. Taking into account these formulations, the charge-state distribution of the plasma ions existing in various ionization states through their fractional densities and average charge state has been studied. The local thermodynamic, corona, and collisional-radiative equilibria are considered for the ionization model. Numerical results for Al plasmas with electron temperatures of 0.1{endash}1.0 keV and electron densities of 10{sup 20}{endash}10{sup 22}cm{sup {minus}3} are presented and discussed. It is observed that the several formulations for the ionization and recombination coefficients predict variously their rates. The consideration of these formulations in an ionization model is noted to significantly modify the charge-state distribution of the ions. It is further noted that the corona equilibrium model can be safely applied to laser plasmas with electron densities less than or equal to 10{sup 22}cm{sup {minus}3} for estimating the abundance of high-charge ions relevant to x-ray line radiation studies. {copyright} {ital 1997} {ital The American Physical Society}

Simultaneous assessment of conformation and aggregation of beta-amyloid peptide using electrospray ionization mass spectrometry.

Electrospray ionization mass spectrometry was used to study conformation and aggregation of the synthetic beta-amyloid peptide, residues 1-40 (betaA4), as a function of concentration and sample aging. All mass spectra showed a major envelope of peaks corresponding to charge states of 7-3 of the monomeric form of betaA4. In addition, weaker envelopes of peaks corresponding to charge states of dimeric, trimeric, and tetrameric betaA4 species were seen under gentle ionization conditions. The average charge state of the envelope associated with the monomeric form decreased by ca. 0.5 z as samples were aged, indicating that the relatively open form (likely random coil) of the peptide was modified into the more compact form (likely beta-sheet) as a function of sample aging. The aggregate forms became weaker and ultimately were absent both in the more dilute solutions and in aged aliquots of the concentrated sample. These aggregates were interpreted as assemblies of the random coil form. We interpret our inability to see an ion envelope that can be associated with aggregates of the beta-sheet form to be a consequence of the presumed very compact nature of this form. A model for the formation of betaA4 fibrils is proposed and discussed.